The present invention relates to a method of repairing trailing edge portions of partitions of turbines damaged during use and particularly relates to a method of reestablishing the trailing edge portion to a desired trailing edge configuration for each partition undergoing repair using sinker electrode discharging machining techniques.
Partitions for turbine diaphragms typically become damaged during extended use. For example, in steam turbines, erosion and foreign particle impingement on the partitions result in a change in the area through which the steam passes and improper direction of the steam flow relative to subsequent rotor stages. The damage to the partitions results in loss of turbine efficiency and oftentimes vibration. The critical component of the partition is the trailing edge. During regular scheduled outages, steam turbines are usually overhauled to repair damage to the diaphragm partitions, particularly the trailing edges, and other components.
Reshaping the partitions to the designed and desired trailing edge configuration has been a manually intensive operation requiring removal of the damaged material along the trailing edge, addition of new material by a welding operation and final contouring of the repaired partition to the desired design shape. The final contouring is a manual operation with high associated labor costs. Also, diaphragm repair is the rate limiting step that determines the length of the repair and hence the outage. It will be appreciated that minimum outage times are desirable in order to place the repaired turbine back into service as soon as possible.
The partitions are typically repaired by first removing the damaged trailing edge portions of the partitions. For example, the damaged trailing edge portion is ground from the trailing edge toward the leading edge over a distance. conventionally about 0.5 inches. New metal of the proper contour must then be installed in place of the removed damaged section. Weld material is thus applied to take the place of the removed damaged trailing edge portion. The weld material is typically applied to a thickness greater than the designed thickness to enable the built-up weld material to be smoothly contoured into the designed configuration on both the pressure and suction sides of the partition. The weld material, contouring and finishing for each partition is, however, manually labor-intensive and requires extensive processing time. Because outages are expensive to users of turbines and because skilled labor utilized to contour the repaired partitions is limited and costly, it is highly desirable to provide a more efficient and faster repair method which will result in partitions having repaired trailing edge configurations corresponding to the design configurations, as well as to reduce the time required to repair the diaphragm.
In accordance with a preferred embodiment of the present invention, the diaphragms are removed from the turbine and the damaged trailing edge portions of the partitions identified for repair are removed. For example, the damaged trailing edge portions can be ground to remove those portions, leaving a remaining portion of the partition, i.e., the leading edge and a substantial portion of the original partition between the leading and trailing edges. Weld material is then added to each of the partitions undergoing repair to replace the removed damaged trailing edge portion and reestablish a trailing edge portion. The weld material is added along the pressure and suction sides of the partition undergoing repair in excess of the desired trailing edge configuration.
To efficiently and economically contour the added weld material to the desired configuration of the repaired partition, a dielectric tank for containing a liquid dielectric such as water or kerosene is provided. A gantry-type CNC-controlled sinker electrode discharge machining machine with an articulating head is also provided and located for disposition of the machining head in the dielectric of the tank. The articulating head is linearly displaceable along X, Y and Z coordinate axes and pivotable about those axes. The head includes an interchangeable electrode holder for mounting sinker EDM tools. By sinker EDM tools is meant tools provided in the reverse image of the shape to be machined.
Once the weld material has been applied to the partitions undergoing repair and the diaphragm with the trailing edge portions of the partitions reestablished by the addition of weld material thereto is located within the dielectric tank, the sinker EDM machine is employed to contour the pressure and suction sides of the partitions to the designed configuration and also to refinish the junctures of the repaired partitions with each of the inner and outer bands of the diaphragm. To accomplish the foregoing, and with the diaphragm in the dielectric tank, a first tool is applied to the articulating head. The first tool has a tool surface shape corresponding to the designed shape of the suction side of the trailing edge of the partition. By applying the suction side sinker EDM tool to the suction side of the trailing edge reestablished by the addition of the weld material, the contour of the weld material is shaped and smoothed to form the continuous suction side surface of each partition upon actuation of the EDM machine. Preferably, the suction sides of all of the trailing edges undergoing repair are contoured prior to contouring all of the pressure sides of the trailing edges.
It will be appreciated that because a chill plate was employed on the pressure side as a basis for adding the weld material, the shape of the pressure side of the trailing edge after welding closely approximates the desired final design shape thereof. Consequently, only a very small portion of the added weld material along the pressure side requires removal in order to smooth the weld beads and restore the correct throat area between the repaired and an adjacent partition. Because the throat gap is typically very small, a copper sinker EDM tool having a surface corresponding to the desired surface along the pressure side of the repaired trailing edge is utilized. With the copper plate having a surface shaped to correspond to the pressure side of the repaired trailing edge applied against the reestablished trailing edge with the weld material, electrode discharge machining is performed to shape and contour the pressure side of the trailing edge.
After each pressure side is machined, a third sinker EDM tool having a surface corresponding to the juncture of the suction side and an arcuate surface of the band is deployed. The tool renders the junctures at opposite ends of the partition and the respective inner and outer bands at an angular relation, e.g., generally perpendicular, to one another.
In a preferred embodiment according to the present invention, there is provided a method of repairing a trailing edge portion of partitions forming part of a diaphragm for a turbine, comprising (a) removing a damaged trailing edge portion of one of the partitions, leaving a remaining portion of the one partition in the diaphragm, (b) adding weld material to the one partition to replace the removed damaged trailing edge portion and reestablish a trailing edge portion on the one partition, the added weld material along pressure and suction sides of the reestablished trailing edge portion of the one partition being in excess of a desired trailing edge configuration for the one partition, (c) immersing at least the one partition having the added weld material into a liquid dielectric and (d) using discrete tools on the pressure and suction sides, respectively, of the one partition while the one partition and tools are immersed in the liquid dielectric, electrode discharge machining the pressure and suction sides of the reestablished trailing edge portion to a predetermined shape to form the desired trailing edge configuration.
In a further preferred embodiment according to the present invention, there is provided a method of repairing trailing edge portions of partitions forming part of a diaphragm for a turbine, comprising (a) removing damaged trailing edge portions of the partitions requiring repair, leaving remaining portions of the partitions undergoing repair in the diaphragm, (b) adding weld material to the remaining portions of the partitions to replace the removed damaged trailing edge portions and reestablish trailing edge portions therealong, the added weld material along pressure and suction sides of the reestablished trailing edge portion of the one partition at least corresponding generally to a desired trailing edge configuration for the one partition, (c) immersing the diaphragm with the partitions having the added weld material into a liquid dielectric and (d) using a tool on each of the pressure and suction sides, respectively, of the partitions having the added material and while immersed in the liquid dielectric, electrode discharge machining the pressure and suction sides of the reestablished trailing edge portions to predetermined shapes to form the desired trailing edge configurations.